1. Field of the Invention
The present invention relates to a camera, having a target follow up function, which automatically follows up a moving object to be photographed.
2. Related Background Art
As a device which automatically follows up an object to be photographed which is moving, and which performs focus detection and focus adjustment, an automatic follow up focus detection device is known (refer to U.S. Pat. No. 5,031,049). The overall construction of this device is shown in FIG. 8. A ray bundle from the object to be photographed which has passed through a photographic lens 20 is led to and focused upon an imaging element 21 which is made up from a plurality of charge accumulation type photoelectric conversion elements. The imaging element 21 outputs object to be photographed image signals which correspond to the distribution of light intensity in this image of the object to be photographed to a signal processing circuit 22 in time series. The signal processing circuit 22 converts these object to be photographed image signals into a video signal and outputs them, and at the same time outputs color difference signals (R-Y) and (B-Y) to a color detection circuit 23. This color detection circuit 23 detects color information relating to the object to be photographed, and a follow up circuit 24 records this detected color information as characteristic information of the object to be photographed. Further, this follow up circuit 24 compares this color information which is recorded as characteristic information of the object to be photographed and the color information newly detected by the color detection circuit 23, and based upon the result of this comparison determines whether or not movement of the object to be photographed has occurred. And, when the target object to be photographed has moved, the follow up circuit 24 moves the focus detection region to the position to which said target object has moved, and performs focus detection and focus adjustment.
However, when an attempt is made to apply the above described prior art for follow up of an object to be photographed to a single lens reflex camera which uses silver halide film, since an electrical signal representing an image of the object to be photographed for performing follow up cannot be obtained from the silver halide film, it is necessary to provide a separate imaging element for follow up of the object to be photographed.
However, since a single lens reflex camera is constructed so that a ray bundle from the object to be photographed which has passed through a photographic lens is reflected from a quick return mirror and is directed to the eyeball of the photographer via an optical system including a pentaprism or the like provided in the upper portion of the camera, it is necessary to provide the photometric element at a position after the light from the object to be photographed has passed through the pentaprism. Further, with a single lens reflex camera, in order for a light bundle from the object to be photographed which has passed through the photographic lens to reach the surface of the film directly during the operation of exposure, the quick return mirror is raised and is removed from the light path between the photographic lens and the surface of the film at this time. Accordingly, if the imaging element is provided at a position after the light from the object to be photographed has passed through the pentaprism, during the exposure process detection of an image of the object to be photographed by the imaging element is temporarily interrupted. As a result, if the object to be photographed continues to move during the exposure process as well, after the exposure has been completed the object to be photographed is lost sight of, and the problem arises that continued follow up becomes impossible.
Further, a correlation follow up device is per se known (refer to Japanese laid-open Application No. 56-86594) which performs correlation calculation between two sets of image data representing the object to be photographed which are detected at different time points by the imaging element, and which follows up the target object to be photographed within the visual field of the imaging element. However, with this device, there is no consideration of any measures to be taken when the target object to be photographed is lost sight of.